A Sharper Look at Quantum Fluids of Light

A Sharper Look at Quantum Fluids of Light
A Sharper Look at Quantum Fluids of Light - Figure 1: Outline of the "pump probe" setup used by Claude and colleagues to characterize a polariton fluid in a semiconductor cavity. A "pump" laser pulse (red) produced polaritons via photoexcitation. The team obtained the polariton distribution curve by measuring the gap reflectance of a "probe" pulse for different angles of incidence.

Research into quasiparticle dynamics in silicon microcavities provides previously unheard of insights into the motion of quantum light fluids.

Superfluidity, or the capacity of a fluid to move without resistance, is not just a property of hydrodynamically characterized systems.

The discovery that light traveling in a nonlinear medium can exhibit quantum hydrodynamic properties sparked optics researchers' interest in superfluids and other quantum fluids more than a decade ago.

Propagating geometries, in which photons are trapped in semiconductor micro-voids and photons move through a bulk material, have emerged as two platforms for studying these "fluids of light."

Both arrangements allow the photons to gain effective mass and interact effectively with each other, which can cause them to act as a quantum fluid as a whole. In particular, difficulties in investigating the collective excitations that characterize quantum fluid behavior limit our ability to fully grasp these unusual states. A quantum fluid composed of polaritons – quasiparticles produced by the strong coupling of photons and excitons in a semiconductor micro-void – has now been characterized in unprecedented detail by Ferdinand Claude of the Sorbonne University Kastler-Brossel Laboratory (LKB) in France and colleagues. The strategy they developed shows potential for investigating new quantum-fluid regimes, some of which can be used as gravity-analog models.

Microcavities in semiconductors provide an excellent platform to study photon-hydrodynamic effects.

When such a space is illuminated by electromagnetic waves whose frequency matches the space resonance, the component of the wave vector perpendicular to the space plane is quantized. The relationship between this wave vector component and the photon frequency consequently shows a quadratic dependence, which gives the photons an effective mass. Laser illumination also induces bound hole-electron states known as excitons. Polaritons, a type of quasiparticle combining the properties of photons and excitons, are created as a result of coupling between photons and excitons in space. These polaritons interact via exciton-exciton coupling, and their masses are determined by the effective masses of the exciton and photon.

Thus, they can collectively act as a stream of large, interacting particles or a quantum fluid. Polaritonic systems have been shown over the past decade to exhibit quantum fluid properties, including Bose-Einstein condensation and superfluidity.

Beyond this qualitative definition, two-dimensional quantum fluids and void polaritons share a mathematical framework known as the Gross-Pitaevskii equation, which governs how both systems evolve over time. The presence of collective excitations, i.e. small density disturbances that propagate to its surface when the fluid is at rest, is a defining feature of quantum fluid activity.

A distribution relationship similar to Bogoliubov's is used to characterize this spread; It has a sound-like region (a linear energy-momentum coupling) at large length scales and a free particle region (a parabolic relationship) at small length scales. The primary goal of Claude and colleagues is to measure these collective excitations, also called Bogoliubov waves.

Since polaritons produced by laser photoexcitation and having a finite life are non-equilibrium particles, cavity-polariton systems require expansions of Bogoliubov theory. This distinction presents difficulties in collecting and using experimental data.

Using a "pump" laser to excite polaritons, researchers can measure Bogoliubov excitations by observing the photoluminescence caused by the decay of polaritons. In previous studies, the pump laser frequency was much higher, making it easier to distinguish pump photons from photoluminescent photons. However, various polaritons, some of which are not a component of the quantum fluid, are produced by non-resonant excitation. Observed spectra are distorted by their presence, especially in areas where signs of superfluidity are expected. An alternative strategy is to use a pump near or above resonance to illuminate the cavity. However, the energy resolution of this method makes it difficult to see many fine details of the Bogoliubov distribution curve.

The group previously created a new coherent probe spectroscopy-based method that allows researchers to go beyond these limitations. In the method, an adjustable laser field that probes Bogoliubov excitations comes after the pump pulse. The system can achieve polaritonic fluid properties with remarkable spatial and spectral precision, thanks to the probe laser's capacity to separate the signal from the fluid's background emission.

Claude and colleagues provided a comprehensive characterization of the distribution of collective excitations of polaritonic fluid through a series of studies. The reflectivity of the cavity was determined for a given pump energy at various probe beam slopes (Fig. 1).

When the probe resonated with the collective polariton excitations, the reflectivity decreased at each angle. This allowed the researchers to characterize Bogoliubov excitations with various wave vectors and then reconstruct the dispersion relationship. They also took advantage of the Gaussian shape of the rays to fit their experimental findings into theories that account for such shapes, allowing them to determine the speed of sound in polaritonic fluid.

The Bogoliubov distribution relationship in a superfluid has two branches: a normal branch with a normal distribution and a phantom branch with a negative distribution. The name of the second branch comes from how difficult it is to stimulate it and then observe it. LKB researchers have already solved this experimental challenge and, thanks to the extraordinary sensitivity of the pump-probe assembly, have discovered traces of this phantom branch in reflectance measurements of their cavities. The new approach significantly improves the characterization of both branches, particularly for areas of the dispersion curves that had previously received insufficient attention, such as those corresponding to small wavenumbers.

The team was also able to characterize the onset of different fluid instabilities and noticed new details on how fluid density and other properties affect the speed of sound.

This work adds a degree of experimental control to the quantum fluids framework, opening the door to more comprehensive quantitative investigations of polaritonic fluids. This arrangement will provide hitherto unattainable insights into quantum hydrodynamics by examining minute variations from the norm in quantum fluid behavior. It could also make it possible to recreate hard-to-study phenomena related to astrophysics, cosmology and quantum gravity using polaritonic systems as optical analogs of gravity.

Source: physics.aps.org/articles/v16/74


Günceleme: 09/05/2023 14:16

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